The present invention relates generally to printing systems and more particularly to duplex printing systems for printing variable information on one or both sides of a sheet.
Apparatus for duplex copying of documents and for duplex printing by means of laser printers are known in the art. U.S. Pat. No. 4,949,949 to Holmes et al. describes a xe2x80x9cHybrid Sequencing Duplex Automatic Document Handling Systemxe2x80x9d which includes apparatus for handling document sheets both sides of which are to be copied and for making duplex (i.e. double-sided) copies of such document sheets. The apparatus involve the use of one or more pairs of reversible rollers, lengthy inversion paths, and buffer trays for the handling of the documents and the copy paper prior to and in the course of making duplex copies. U.S. Pat. No. 4,884,794 to Dinatale et al. describes a document handler for duplex photocopying having first and second inverting path segments, which are utilized to re-orient the copy paper prior to duplex copying. U.S. Pat. No. 5,003,355 to Tanzawa describes a sheet transport control apparatus for use in a duplex unit of a laser printer, the apparatus including a transport system and a switchback system, and a series of driving motors and sensors. All these systems described in the prior art share the common feature of being mechanically complex, and they all involve transporting the paper through relatively lengthy and convoluted paths after printing on the first side so as to be able to print on the second side. Other systems for duplex printing are described in U.S. Pat. Nos. 4,806.079; 4,814,822; 4,568,169; 4,639,126; 4,428,667; 4,607,940; 4,375,326 and 5,020,788 and EP publication 0342704.
PCT publication 93/04409 describes a switchback system with a much shorter path than older systems, which allowed for on demand duplex printing without storage of large numbers of sheets.
Systems which utilize the same impression roller and/or the same printing engine for printing both sides of a web are known in the art. However, even in those systems the two sides of the web are printed at different printing positions in the printer and the web is not indexed at an edge.
Also known are systems for reversing sheets between printing stations. One such system is called a xe2x80x9cperfectaxe2x80x9d type system and comprises a roller that acts to turn over the sheet. Such systems, unlike those used for laser printers, reference the printing sheet from the same edge for printing on both sides.
A prior art perfecta system 10 is shown in FIGS. 1A and 1B. This system comprises a first impression roller 12, which holds a sheet 14 for printing thereon by a print roller (not shown). Sheet 14 is transferred to roller 16 where it is held by a front edge clamp 20. Roller 16 continues to rotate and the front edge of sheet 14 passes an inverting roller 18. When the trailing edge reaches inverting roller 18, a clamp 20 on roller 18 catches the trailing edge of sheet 14 and, as shown in FIG. 1B inverts the sheet prior to its being clamped to a second impression roller 22.
An advantage of perfecta systems is that while the leading edge for printing the first and second sides of the sheet are reversed, the same edge is used as a reference position for printing both sides. Another advantage of perfecta systems, which is related to the first advantage, is that the sheets are always positively held by the system during inversion of the sheet. Positive holding of sheets distinguishes xe2x80x9cperfectaxe2x80x9d systems from systems which utilize a single printing engine and which generally do not positively hold the sheets during the entire process of transfer and reversal.
However, inverting systems which provide the advantages of perfecta systems are not known in a printer using the same impression roller and printing engine for printing both sides of the sheet.
One aspect of some preferred embodiments of the present invention provides apparatus and a method for duplex printing of sheets, utilizing the same edge of the sheet for reference for printing both sides thereof, while utilizing the same impression roller and/or the same printing engine.
One aspect of some preferred embodiments of the present invention provides apparatus and a method for duplex printing of sheets utilizing an impression roller for printing both sides of a sheet, while positively holding the sheet during the entire process of reversal and transfer of the sheet. Preferably, this means that the sheet is positively held from the start of the printing process to its end.
In a preferred embodiment of the invention only one sheet, other than a sheet held in the impression roller is in the inverting system.
In a preferred embodiment of the invention, the same set of one or more printing engines is used in the printing of both sides of all the sheets.
In preferred embodiments of the present invention a perfecta-like system is used. This system includes rollers and/or belts which receive the sheet from one position on the circumference of an impression roller and, after reversing the sheet. delivers the sheet to a second position on the impression roller. Preferably, the path traveled by the sheet between the two positions holds an integral number of sheets. Preferably, the impression roller holds a plurality of sheets and presents them seriatim to one or more print engines. Preferably, the engine or engines are electrographic or other engines providing programmable images such as electrophotographic engines, ink or bubble jet print heads thermal printing heads or any other suitable printing engines.
Other aspects of some preferred embodiments of the invention are concerned with high speed printing engines, especially with high speed electrographic printing engines. In such engines special care must be taken in charging a photoreceptor and, when liquid toner is utilized, in treating and transport of the image. Some aspects of some preferred embodiments of the present invention deal with improvements in such engines especially useful for high speed printing.
There is thus provided, in accordance with a preferred embodiment of the invention duplex printing apparatus for printing on two sides of a sheet, the apparatus comprising:
an impression roller on which the sheet is held during printing;
imager which prints an image on a first side of the sheet while it is being held on the impression roller; and
a sheet inverter which removes the sheet from the impression roller, inverts the sheet and returns it to the impression roller for printing on a second side of the sheet by the imager, wherein the sheet is held on said impression roller referenced to a first edge thereof during the printing of the first side thereof and is also held on the impression roller referenced to said first edge during printing of the second side thereof.
Preferably, the sheet inverter positively controls the position of the sheet during the inversion thereof, without releasing the sheet during the inversion.
There is further provided, in accordance with a preferred embodiment of the invention a duplex printing apparatus for printing on two sides of a sheet, the apparatus comprising:
a surface, on which an image to be printed is selectably formed;
an impression roller on which the sheet is held during printing, referenced to a first edge thereof;
an imager which prints an image on a first side of the sheet while it is being held on the impression roller; and
a sheet inverter which removes the sheet from the impression roller, inverts the sheet and returns it to the impression roller for printing on a second side of the sheet by the imager, wherein the sheet inverter positively controls the position of the sheet from the removal of the sheet from the impression roller to the return of the sheet thereto after the inversion thereof, without releasing the sheet.
in a preferred embodiment of the invention the sheet inverter comprises:
a paper pick-off system which removes the sheet from the impression roller, after printing of the first side of the sheet, while the sheet is held referenced to said first edge;
an inverting transport past which the first edge is carried while the sheet remains referenced to said first edge; and
a sheet pick-off on said inverting transport which captures a second edge of the sheet, opposite the first edge while the sheet is still being held referenced to the first edge, such that said capture is made referenced to the first edge,
said inverting transport transporting the second edge to the impression roller for capture by the impression roller, such that the second side of the sheet is presented for printing by the imager.
Preferably, the apparatus includes at least one intermediate transport which receives the sheet from the sheet pick-off system and transports it to the inverting transport while the sheet remains referenced to the first edge. Preferably, the at least one intermediate transport comprises at least one roller. Preferably, a sheet path in the paper pick-off, sheet pick-off and intermediate transport is at least the length of a plurality of sheets.
In a preferred embodiment of the invention, the inverting transport comprises a transport roller.
Preferably, the page inverter comprises a perfecta system.
In a preferred embodiment of he invention, the page inverter stops the motion of the sheet while positively holding it referenced to the printing on the first side and then moves the sheet in a reverse direction for capture by the impression roller.
Preferably, the paper pick-off comprises at least one vacuum pick-off that picks the sheet off the impression roller and holds it while it is being inverted. Preferably, the at least one vacuum pick-off comprises two sets of vacuum pick-offs each comprising at least one pick-off, wherein the vacuum pick-offs pick offs alternate in picking sheets off the impression roller.
Preferably, the apparatus includes a belt transport that receives a sheet from the at least one vacuum pick-off and transports it to the impression roller while positively holding the sheet referenced to the image printed on the first side.
Preferably, the vacuum pick-offs rotate about an axis and have a radial extent from the axis and the apparatus has at least one at least partial disk concentric with the axis and the disk has a radial extent equal to the radial extent of the vacuum pick-offs.
In a preferred embodiment of the invention, the impression roller and the inverting system hold no more than two sheets at any one time.
Preferably, the imager comprises a plurality of imaging stations each of which transfers an image of a different color to the sheet.
In a preferred embodiment of the invention, the imager includes an image forming surface on which the image is formed prior to transfer to the sheet. Preferably, the imager includes at least one intermediate transfer member to which images are transferred from the image forming surface and from which the images are transferred to the sheet.
In a preferred embodiment of the invention the imager provides different images to the sides of the sheet.
According to one preferred embodiment of the invention the imager is an electrographic imager.
The imager can be a powder toner imager or a liquid toner imager.
The imager can be an ink-jet or bubble jet imager.
In a preferred embodiment of the invention the impression roller is adapted to hold a plurality of sheets at one time.
There is further provided, in accordance with a preferred embodiment of the invention, a duplex printing method for printing on two sides of a sheet, the method comprising:
printing an image on a first side of the sheet at a printing position, the sheet and thus said printing being referenced to an edge of the sheet;
inverting the sheet and returning it to the printing position while it remains referenced to said edge; and
printing an image on a second side of the sheet at said printing position while the sheet and thus said printing is referenced to said edge.
Preferably, the position of the sheet is positively controlled during inversion thereof, without releasing the sheet between printing of the first and second sides thereof.
There is further provided, in accordance with a preferred embodiment of the invention a duplex printing method for printing on two sides of a sheet, the method comprising:
printing an image on a first side of the sheet at a printing position;
inverting the sheet and returning it to the printing position; and
printing an image on a second side of the sheet at said printing position,
wherein the position of the sheet is positively controlled during printing and inversion thereof, without releasing the sheet.
Preferably, the sheet is delivered to said printing position by a moving member on which it is held while being referenced to said edge.
In a preferred embodiment of the invention the sheet is printed while being moved by the moving surface, past the printing position and wherein the sheet is held at said edge during printing of one side thereof and held by an opposite edge of the sheet during printing of the other side thereof.
Preferably, the sheet is printed while being moved past the printing position with said edge passing the position first during printing of one side of the sheet and wherein said edge passes the printing position after the rest of the sheet during the printing of the other side of the sheet.
Preferably the method includes printing different images on the two sides of the sheet.
In a preferred embodiment of the invention, inverting the sheet includes:
moving the sheet to a first position while holding it referenced to the first edge; and
stopping it at the first position: and
returning it to the printing position while it remains referenced to the first edge.